This invention relates to electron guns, and especially to electron guns for use in television picture tubes. The invention is particularly directed to electron lenses for such guns.
It is well known that spherical aberration in an electron lens can be desirably reduced by making the field of the lens weaker and extending it over a greater length along the path of the beam. It is also well known that one type of lens for doing this is the resistive lens wherein a plurality of metal electrode plates are arranged in serial fashion, and a voltage gradient is established along the lens by applying different voltages to the different plates by way of a resistive bleeder element provided within the vacuum envelope of the electron tube itself.
The prior art has disclosed various forms of plural-plate resistive lenses, for example: FIG. 1 of U.S. Pat. No. 2,143,390 issued to F. Schroter on Jan. 10, 1939; U.S. Pat. No. 3,932,786 issued to F. J. Campbell on Jan. 13, 1976; and FIG. 3 of U.S. Pat. No. 4,091,144 issued to J. Dresner et al on May 23, 1978. Although Schroter shows the bleeder resistor only schematically, Campbell discloses a practical embodiment of a bleeder resistor disposed on a glass support rod (bead) of the electron gun structure, and Dresner et al shows a practical embodiment of a stack of alternate metal electrodes and insulator blocks with a resistive bleeder coating applied along one edge of the stack. However, in practice, the Campbell structure requires many connectors to make contact between the series of apertured electrodes and the bleeder resistor, and moreover increases the likelihood of cracked beads during fabrication due to the large number of electrodes embedded in the glass beads. Furthermore, both the Campbell and Dresner et al lenses depend for their field accuracy upon the uniformity of the resistive bleeder coating, the fabrication of which is very difficult to control.
An improvement over the Dresner et al structure is disclosed in U.S. Application Ser. No. 51400 filed June 25, 1979 by B. Abeles, and incorporated herein by reference for purposes of disclosure. The Abeles lens structure comprises a plurality of apertured electrodes and resistive spacer blocks alternately stacked and brazed together to form an electrically continuous structure. The resistive blocks comprise insulator blocks which, prior to being assembled into a unitary stack with the apertured electrode plates, are each coated along at least a portion of one surface with a suitable resistive material. Such precoating (i.e. coating prior to assembly) of the blocks allows them to be pretested before assembly and sorted according to their resistivity characteristics. The Abeles construction has proved to be electrically and mechanically acceptable, but involves relatively high costs entailed in the brazing together of the blocks and plates.